Ligaments are strong fibrous soft tissue connecting the articular ends of bones to bind them together and to facilitate or limit motion. Injuries to ligaments are common, and patients who are physically active are generally more susceptible to such ligament injuries. The anterior cruciate ligament (ACL) of the knee joint is a ligament frequently injured by such patients. ACL injuries cause instability in the knee joint which, when left untreated, may lead to degenerative arthritis. Because of this condition, ACL reconstruction may be required. Generally during ACL reconstruction, a substitute soft tissue ligament or graft is attached to the femur (femoral fixation) and/or to the tibia (tibial fixation) to facilitate regrowth and permanent attachment.
There are several known methods for performing ACL reconstruction, and there are also several tibial or femoral fixation devices that may be used with these methods.
In surgery it is generally known to use soft tissue tendon grafts (e.g. hamstring tendon, taken from the thigh of the patient) to replace the severely damaged ACL. In a typical surgical procedure one end of a soft tissue graft is fixed into a drill hole made from the knee joint into the distal femur and another end of the graft is fixed into a drill hole made into the proximal tibia. The ends of the graft are fixed into the drill holes with fixation screws and in most cases with so-called interference screws. An interference screw may be a screw that has a larger diameter (including any grafts or tendons) than the cavity, thus generating a force that holds the tendon. A screw is installed into the space between the drill hole and the soft tissue grafts to lock the grafts into the drill hole. The tendon then acts as a new ACL.
There are several known methods for performing ACL reconstruction, and there are also several tibial or femoral fixation devices that may be used with these methods. The fixation screws, like interference screws, are normally made of metal like stainless steel or titanium, or of a bio-absorbable polymer like polylactide. An interference screw may be considered as metallic and/or bio-absorbable polymeric materials and composites, which are suitable for manufacturing of tendon graft fixation screws, are well known in the art, for example as described in the literature.
Conventional extra-articular hamstring graft fixation techniques have complications, such as suture stretch, graft tunnel motion and so-called windshield wiper effect where the size of the intra-articular drill hole end will increase due to graft movement in the drill-hole. Also the use of screws as fixation implants for soft tissue grafts in anterior crucial ligament procedures is complicated due to: 1) the threads of the screw cutting the grafts during screw installation if the screw is too big in relation to the tendon and/or if the space between the drill hole and tendon grafts is too small; 2) the threads of the screw damaging the tendon during screw installation; 3) the tendon rotating with the screw during screw installation so that the optimal position of the grafts is lost and/or the grafts are damaged; 4) divergence of the grafts and/or screw occurring; and 5) the bio-absorbable screw breaking during insertion.